1. Field of the Invention
This invention relates to spatial power combiners, such as quasi-optic grid arrays, and in particular to architectures for efficiently removing heat generated by, and supplying bias to, spatial power combiners.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the generation and transmission of radio frequency (“RF”) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called “power combining” has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at high frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, “spatial power-combining” was proposed as a potential solution to these problems. In spatial power-combining, the individual outputs of microwave and millimeter wave solid-state devices combine in free space to provide a single combined power output more effectively than can ostensibly be achieved using the above-described conventional power-combining techniques.
As used herein, the term “spatial power-combining” broadly applies to all structures that couple component outputs in free space. A subset of spatial power-combining that has recently received significant attention is called “quasi-optical power combining.” The theory underlying “quasi-optics” is that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, thus minimizing conductor losses. Furthermore, monolithic fabrication enables potentially thousands of microwave or millimeter wave active devices to be incorporated on a single wafer.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the filed has been explosive.
A quasi-optical array amplifier includes a two-dimensional sheet of active devices, interconnected with patterned conductors, that accepts a polarized electromagnetic wave as an input and radiates an amplified output wave with a polarization that is orthogonal to the input polarization. Two array amplifier configurations that have been previously reported are transmission-mode arrays and reflection-mode arrays. FIG. 1 shows a typical transmission-mode grid amplifier 10, wherein an array of closely-spaced interconnected differential pairs of transistors 14 are formed in a grid-like pattern on the front surface of a semiconductor chip 12. The chip is sandwiched between an input polarizer 18 and an output polarizer 24. An input signal 16 passes through the horizontally polarized input polarizer 18 and creates an input beam incident from the left that excites rf currents on the horizontally-patterned, polarized, metal input antennas 20 of the grid on the chip 12. These currents drive the inputs of the transistor pairs 14 in the differential mode. The output currents are redirected along the grid's vertically-patterned polarized antennas 22, producing, out the right side of the array, a vertically polarized output beam 30 via an output polarizer 24.
Reflection-mode arrays also use two-dimensional active grids similar to the active grids in transmission mode arrays. However, reflection-mode arrays require that the EM radiation have access to only the front (active grid) surface of the chip The back surface of the chip is mounted on a dielectric substrate atop a metallic mirror.
Unfortunately, drawing heat away from quasi-optical arrays and, more generally, from any type of spatial power combiners remains a problem, especially for high-power, high-frequency applications. In microwave and millimeter wave semiconductor devices, heat is generated in the circuitry fabricated on the front (or top) surface of the semiconductor. In conventional microwave and millimeter wave circuits, in order to enhance heat removal, the semiconductor wafer is typically thinned to a final thickness of 50 to 150 microns. Further, the back (or bottom). surface of the semiconductor chip is completely covered with a metal “ground plane,” which acts as an equipotential shield and return conductor to the power supply, that is electrically connected to the devices on the front surface through metallic “vias.” Additionally, this metal layer provides both a mechanical support as well as a surface for soldering devices to a metalized carrier, thus enabling a high thermal conductivity interface between the device and its package.
In quasi-optical grid arrays, the microwave/millimeter wave signals radiate into or are absorbed from fields above and below the semiconductor. Covering the back surface of the semiconductor with metal would impair the coupling of radiation fields to the patterned conductor on the front surface of the semiconductor, and would completely prevent the transmission of the radiated signals through the semiconductor wafer.
Previous implementations of monolithic grid arrays have been fabricated on semiconductor chips without metal ground planes disposed on their back surfaces. As seen in FIG. 1, in order to draw the substantial amount of heat generated in the circuitry away from the grid array, the back surface of the chip is generally mounted on a dielectric heat spreading slab 13. This dielectric slab is typically attached to the bottom, or back surface of the semiconductor chip 12 using an electrically non-conductive adhesive. Further, DC power supply to the active devices (biasing) has been carried out using the patterned metal lines placed exclusively on the front surface of the semiconductor chip. Unfortunately, having all of the conductors on the front side of the chip limits the efficiency of the biasing, and the absence of backside metal precludes the use of high thermal conductivity attachments using a solder or traditional eutectic die-attach.
Thus, there is a definite need for an improved means for removing heat generated by active devices in spatial power combiners, such as quasi-optical grid arrays, especially where higher output powers are required. There is also a need for improved methods and structures that allow for more flexibility in the placement of bias and tuning elements.